For reading/writing data from/to an optical disc, the optical head is moved in two directions, i.e. a direction perpendicular to the disc face, which is referred to as a focusing direction, and a direction parallel to the disc face, which is referred to as a tracking direction. Meanwhile, the light emitted by a light source such as a laser diode is focused by an object lens of the optical head on the optical disc, and the light reflected by the optical disc is transmitted to a light sensor to extract data. According to the obtained data, a focusing error signal and a tracking error signal can be realized for further adjusting the movement of the optical head in the focusing direction and the tracking direction.
To find the perfect focusing position by using the focusing error signal, a variety of methods such as astigmatic method, spot-size method, Foucault method, etc. can be employed for focusing control. Hereinafter, an astigmatic method is described in more detail as a focusing control example for better understanding. For implementing the astigmatic method, the optical sensor of an optical disc drive includes four light receiving parts A, B, C and D for respectively receiving the main beam reflected from the disc, as can be seen in any of FIGS. 1A˜1C. The summation of the light intensities reflected from the receiving parts A, B, C and D is defined as a data signal HF=A+B+C+D. During a tracking operation, a radial push-pull signal PP=(A+B)−(C+D) is generated. On the other hand, as far as a focusing operation is concerned, the focusing error signal FE is substantially a difference between the summation of the overall light intensity received by the receiving parts A and C and the summation of the overall light intensity received by the receiving parts B and D, i.e. (A+C)−(B+D), where A, B, C and D are light intensities received by the regions A, B, C and D, respectively. FIGS. 1A˜1C illustrate three kinds of focusing results. When the light emitted by the light source is perfectly focused on the desired point, as shown in FIG. 1B, the overall light intensity received by the receiving parts B and D will be equal to that the overall light intensity received by the receiving parts A and C, i.e. FE=(A+C)−(B+D)=0. In another case shown in FIG. 1A, the value of (A+C)−(B+D) is minus, which indicates a focusing position above the perfect position. On the other hand, in the case shown in FIG. 1C, the positive value of (A+C)−(B+D) indicates a focusing position below the perfect position. The relationship between the voltage of the focusing error signal FE and the depth of the focusing position (or the distance of the focusing position from the lens), which is so-called as “S-curve”, is illustrated in FIG. 1D. The astigmatic method is performed in a closed-loop focusing control manner to zero the focusing error signal FE, thereby locating the perfect focusing position.
Due to the spherical shape of the lens of an optical head, the focusing of the lens would be less than ideal. Therefore, spherical aberration, which is some kind of image imperfection that occurs due to the increased refraction of the laser rays that occurs when rays strike the lens near its edge, would be rendered. To remedy the spherical aberration, a collimator is introduced upstream of a lens of the optical head to filter the rays so that only those traveling parallel to a specified direction can pass through.
For compensating spherical aberration as well as focusing offset, a collimator is combined with a lens to be included in an optical head of an optical reading/writing apparatus such as a CD, DVD, Blu-ray and HD-DVD. Please refer to FIG. 2 which schematically exemplifies the laser rays emitted by a laser diode 20 and processed by a collimator 21 and a lens 22 to be well focused on an optical disc 23. As shown, parallel laser rays are obtained through the collimator 21 so as to be precisely focused on the optical disc 23 by the lens 22.
Due to the introduction of collimator 21, one-dimensional compensation is insufficient for locating the optimal focusing position of the optical head. Accordingly, the collimator 21 and lens 32 are both adjusted in a two-dimensional manner to locate the optimum focusing position on the optical disc 23. With the movement of the collimator 21 and lens 22, the summation of the overall light intensities received by the receiving parts A, B, C and D of the optical head changes. Thus the data signal HF is generated and a jitter of the data signal HF-jitter is obtained.
For a written disk, the calibration of the focus offset and spherical aberration can be performed based on the data having been present in the disk. On the other hand, for an empty optical disk that includes no data thereon, calibration of focusing offset and spherical aberration can be performed with the information recorded in the blank optical disk during the OPC (Optimum Power Calibration) procedure. OPC is a function of an optical disk recording/reproducing apparatus that checks and calculates a proper writing power and reflection of an optical disk in use so as to make proper adjustments for writing the optical disk. With the determined optimum power, a few tracks are written in a drive calibration zone on the disk (normally OPC-area). Since there has been written information on this area, focus offset and spherical aberration can be calibrated based on the HF-jitter. Then an optimum combination of focus offset and spherical aberration resulting in the best writing quality, e.g. the lowest HF jitter, is used for compensation in subsequent reading/writing operations.
The above calibration method, however, has some drawbacks. In practice, the HF-jitter is expressed as a function of spherical aberration relevant to the position change of the collimator and focusing offset relevant to the position change of the lens. Usually, the algorithm repeatedly changes the focusing offset until a sufficient wide dynamic range is found containing or predicting the optimum value. Sometimes the focusing offset is set to such a large value that the servo loses focus. That invokes a focusing recovery. With too many recoveries, focusing offset optimization would fail. Also with a too large focus-offset normal playback and recording become unacceptably unstable. Therefore, these recoveries should be prevented as much as possible. Aside from, as the sample quantity of the information recorded in the OPC area during the OPC procedure is not enough for completing the calibration of the focusing offset and spherical aberration, the optical head with the lens and collimator need be moved to and forth to obtain sufficient information. The calibration process is not efficient.